what is a good seafood shop in hongkong?

If you mean shops that you can buy seafood material, I suggest you go to Sai Kung District or Sheung Wan (near Sheung MTR Station). Seafood in these two places are cheap, but with good qualities.
If you want to eat seafood there, I recommend these restaurants: Lei Yue Mun Seafood Restaurant, Fook Lam Moon, Bubba Gump Shrimp.